The present invention relates to poly(butylene terephthalate) injection molded articles which contain one or more agents which render them electroconductive, and to compositions of matter useful for the production of said articles.
Poly(butylene terephthalate) is a plastic which has long been in commercial use because of its excellent resistance to chemicals, its great strength, and its resistance to fracture. In addition, the resin possesses extremely high electrical resistivity (10.sup.15 -10.sup.16 ohms-cm), so that molded articles based on this resin find wide use where this property is important for example as housing for industrial, household and laboratory electrical motors and electrical devices including handtools and radio and television cabinets, and for insulating coatings on wires. Such articles usually contain one or more modifying agents, for example one or more rubbery polymers, to improve the resistance of the articles to impact; one or more reinforcing agents such as glass and nylon fibers to improve their tensile strength; stabilizes, for example, ZnO-ZnS mixtures, to improve the resistance of the articles to heat; and fire-retardants. Such agents have little or no effect on the electrical conductivity of the articles. Additional additives which can be present in molded articles based on poly(butylene terephthalate) are disclosed in copending application Ser. No. 957,801 filed on Nov. 6, 1978 by S. C. Cohen et al., which is hereby incorporated by reference.
In certain commercial and industrial applications it is desirable to provide a plastic injection molding composition which is electrically conductive. Heretofore, it is known that certain organic compounds, notably the condensation products of the higher fatty primary amines with ethylene oxide, are effective internal anti-static agents, but these agents have been found effective only in certain vinyl polymers, and it is generally believed that these agents are not effective in engineering plastics (see Modern Plastics Encyclopedia 1977-1978, pp. 145-146, McGraw-Hill, Inc., New York, N.Y. copyright 1977), hereby incorporated by reference.
It is also known (Maddock U.S. Pat. No. 2,379,976) that electrically conductive plastics based on vinyl chloride resins and plasticizers can be formulated by including in the composition from 30 to 40% by weight of finely divided carbon black. However, Maddock acknowledges that some sacrifice in the physical properties of the plastic result from incorporating such amount of carbon black, and that amounts of carbon black as low as 20% by weight are ineffective for the patentees purpose. Maddock also points out that electrically conductive compositions are known which contain natural or synthetic rubber and carbon black but that it is difficult in this manner to obtain compositions which are both conductive and elastic.
It has now been found that molding and extrusion compositions based on poly(butylene terephthalate) can be made electrically conductive so as to resist accumulation of static electricity by incorporating in the compositions relatively small amounts, i.e., 10 to 25% by weight of the composition, of finely divided amorphous carbon or graphite fibers, in which at least two of the dimensions are smaller than 100.mu.. Moreover, such filled compositions retain to a surprising degree the good physical properties, i.e., tensile strength and impact resistance, of the base poly(butylene terephthalate)polymers.
The further important discovery has been made that certain additives, for example glass fibers, the polycarbonate of bisphenol A, and rubbery polymers which improve impact resistance, which heretofore have been thought to improve only certain physical properties of poly(butylene terephthalate) articles, in preferred instances, very greatly enhance the action of the carbon in rendering the articles electroconductive.
The carbon employed in the composition of the subject invention can be amorphous carbon powder of any of the commercially available type wherein the particles have diameters in the range of 0.1 millimicrons and 100 millimicrons. Preferably the amorphous carbon is pigmentary carbon, for example furnace black, acetylene black, channel black and lamp black, or any of the uncoated carbon blacks which are used as reinforcing agents in the compounding of rubber, wherein the particles have a maximum dimension between 0.2 microns and 2 microns.
Alternatively, the carbon can be in the form of graphite having an equivalent particle size. Graphite powder in the particle size range stated above is suitable, but it is generally preferred that the graphite be in the form of fibers or "whiskers" since in this form the graphite acts both as an anti-static agent and as a fibrous reinforcing agent which improves the tensile strength and impact resistance of the moldings. The preferred fibers have two dimensions in the range of not greater than about 127 microns each, their third dimension being a length not in excess of about 6.4 mm. The carbon can be present in combinations of the forms described above, and thus can be partly in fibrous and partly in powder form.
The action of the carbon in imparting electroconductivity to the compositions of the present invention is fortified by the presence of glass fibers, the polycarbonate of bisphenol A, and rubbery impact modifiers. The glass fibers are of the type customarily employed as reinforcing agents.
The rubbery impact modifier or mixture of modifiers can be a styrene-butadiene block copolymer or a styrene-butadiene-styrene block copolymer or a polyether ester, such as the "Hytrel" rubbers manufactured by duPont, as well as block copolymers having soft segments, such as poly(1,6-hexanediol-azelate-cophthalate) as claimed in application Ser. No. 290,879 filed Aug. 7, 1981, a continuation of application Ser. No. 752,325, filed Dec. 20, 1976. Also suitable as an impact modifier is a multiple stage polymer having a rubbery first stage and an epoxy functional hard final stage as shown in Lane, U.S. Pat. No. 4,034,013; or a multiphase composite interpolymer comprising a cross-linked acrylic first stage which also contains graftlinking monomer and a final rigid thermoplastic phase, as shown in Farnham et al., U.S. Pat. No. 4,096,202. The amounts of these agents, alone or in admixture, are in the range of 2-25% of the overall composition.
In certain preferred embodiments the composition includes electrically inert fillers, especially reinforcing fillers, for example clay, mica, talc and the like, of which clay is preferred. The fillers can be untreated or they can carry a coating of a siloxane or titanate coupling agent.
The glass filaments which are advantageously employed are well known to those skilled in the art and are widely available from a number of manufacturers. The preferred filaments are substantially composed of lime-aluminum borosilicate glass that is substantially soda-free. This is known as "E" glass. The filaments are made by standard processes, e.g., by steam, air or flame blowing or by mechanical pulling. The preferred filaments are made by mechanical pulling and have diameters from about 0.001" to about 0.0001".
The glass filaments which are added to the starting molding composition have any convenient length, for example 1/4" to 2" or more, because they break extensively during the compounding operation. The lengths employed should be those which ensure that the fibers, just before the molding (including extrusion) step, are no longer than about 1/4". Their final length can be as short as 0.0005".
The amount of the aforementioned fillers which can advantageously be present varies widely depending chiefly on the particular polymers and other materials which are present in the composition and on the specifications which the composition must meet, a suitable amount being that which is sufficient to provide a substantial reinforcement. Generally the weight of the reinforcing filler is between about 5% and 50% of the total composition. A suitable amount in any instance is readily found by trial.
Electroconductive articles of low combustibility can be produced according to the present invention by including in the starting compositions an effective amount of a conventional flame-retardant or mixture thereof. As is well known, flame retardants may be based on elementary red phosphorus, phosphorus compounds, halogen, and nitrogen compounds alone or preferably in admixture with synergists such as antimony compounds. Especially useful are the polymeric and oligomeric flame retardants which comprise tetrabromobisphenol A carbonate units (see Wambach, U.S. Pat. No. 3,833,685 which is incorporated herein by reference).
Other ingredients such as dyes, pigments, drip retardants and the like can be added for their conventional purposes.
The compositions may and advantageously will include such other components as heretofore have been present in poly(butylene terephthalate) molding compositions, for example 1% to 50% of polyethylene to improve the flexibility of the moldings, 1% to 50% of a compatible auxiliary polymer, for example poly(ethylene terephthalate) and poly(1,4-butylene isophthalate); mold lubricants; and stabilizers for protecting the molded article from heat.
The compositions of this invention can be prepared by ordinary melt blending procedures, and this is one of the advantages thereof. According to one such procedure, all the components are mixed simultaneously and the resulting batch is extruded to form a rod which is then chopped, providing granules of uniform composition.
It is important to free all of the ingredients, including the carbon, from as much water as possible before the composition is fused or molded.